block: remove the struct blk_queue_ctx forward declaration

This type doesn't exist at all, so no need to forward declare it.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Link: https://lore.kernel.org/r/20210920123328.1399408-12-hch@lst.deSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/blkdev.h

@@ -253,8 +253,6 @@ static inline unsigned short req_get_ioprio(struct request *req)
 
 #include <linux/elevator.h>
 
-struct blk_queue_ctx;
-
 struct bio_vec;
 
 enum blk_eh_timer_return {